The increased expansion of renewable and distributed generation is posing new technical, economic and regulatory challenges to the electricity industry. The growth in renewable energy sources is crucial to meeting electricity sector emissions reduction targets. It is often the case, however, that the renewable resource exists in rural areas supplied by relatively weak distribution networks. Distribution networks were not designed to accommodate high levels of renewable or distributed generation (DG) and so can act as a significant barrier to the connection and operation of DG units. Active Network Management (ANM) is emerging as a preferred solution to the connection and operation of DG units.
ANM concerns the technical challenges that can result from the connection and operation of DG units to distribution networks: power flow management, voltage control and fault level management. ANM has emerged primarily in the UK through the work of the UK Government Embedded Generation Working Group (EGWG), which later became the Distributed Generation Coordination Group (DGCG). An important outcome of one of the work streams of the DGCG was the publication of “Solutions for the Connection and Operation of Distributed Generation” DTI Distributed Generation Programme (Contractor: EA Technology, Authors: Collinson, A., Dai, F., Beddoes, A., Crabtree, J.); KJEL/00303/00/01/REP; 2003. This report is often referred to as the “Basic Active Management” or BAM report and describes solutions to the technical issues of voltage control, power flow management and fault level management for the connection and operation of individual distributed generation (DG) units. The main categories of the solutions proposed for power flow management are pre-fault constraints; post-fault constraints; direct intertripping; generator trip based on power flow measurements and generator power output control based on power flow measurements.
Power flow management based on pre-fault constraints implies the limitation of power flows to that which can be accommodated for the next circuit outage. During normal operation the worst case first circuit outage (FCO) is the N−1 contingency (the loss of the largest of N circuits). Pre-fault constraints represent the traditional approach to connecting and operating DG units and do not constitute ANM. The strategy is commonly referred to as “fit and forget” as it implies that the DG unit will connect up to the N−1 capacity of the network, therefore requiring no operator intervention unless the N−2 contingency occurs. This allows the distribution network operator (DNO) to maintain the passive operation of the system, i.e. the DG unit will not be controlled or required to provide any network support.
Post-fault constraints are applied to a DG unit after an outage has occurred on the network. The BAM report presents three main post-fault constraint strategies: direct intertripping, generator trip based on power flow measurements and generator power output control based on power flow measurements. An example of post-fault constraints using intertripping is provided in the report but this does not address issues associated with real time regulation of DG output based on network constraints. Post-fault constraints can be implemented through direct intertripping of DG units for the tripping of upstream circuit breakers. Communications are required between the branch protection systems and the circuit breaker at the DG site. The reliability of the approach is therefore dependent on the reliability of the communications between sites. On occasions when the direct intertripping scheme is unavailable the DG unit must maintain output within pre-fault constraint levels.
J. Kabouris et al have described a system to facilitate increased connection of wind generation to the Greek transmission network, see ‘Application of Interruptible Contracts to Increase Wind-Power Penetration in Congested Areas’; Power Systems, IEEE Transactions on, Volume: 19, Issue: 3, Pages: 1642-1649, August 2004. The paper distinguishes between guaranteed contracts and interruptible contracts for access to available capacity on 150 kV circuits. Guaranteed contracts can be considered to be consistent with a pre-fault constraint approach. The interruptible contracts for wind generation apply real-time pre-fault constraints. The study looked at extending the capacity for wind farm connections beyond firm/pre-fault constrained levels and managing the output of connected wind farms to ensure power flows remained within firm generation transfer limits. Programmable logic controllers are used to monitor power flows and issue maximum output instructions to the wind farms in a particular area of the transmission network. Both preventive and corrective control actions are considered based on the offline calculation of transfer limits through the congested transmission corridors for certain contingencies. Power flows are limited to be within firm generation transfer limits, despite the installed capacity exceeding this level.
More recently, an ANM system using power flow management has been proposed by R. A. F. Currie, C. E. T. Foote, G. W. Ault, J. R. McDonald; “Active Power Flow Management Utilising Operating Margins for the Increased Connection of Distributed Generation”; IEE Proceedings, Generation, Transmission and Distribution, January 2007. In this, three types of generation are proposed: firm generation, non-firm generation and regulated non-firm generation. Firm Generation (FG) is a term applied to the DG units that have unconstrained access to the distribution network in the N and N−1 state. FG units do not cause the violation of constraints on the network during normal operating conditions or during the FCO. FG capacity represents the traditional approach to connecting and operating DG units. Non-Firm Generation (NFG) are DG units that are connected to the network in addition to FG. NFG are required to curtail output to meet network constraints during the N−1 condition. This is typically performed through intertripping of NFG units from branch protection systems. Regulated Non-Firm Generation (RNFG) units are DG units connected in addition to FG and NFG, the capacity for which is determined in real-time due to load variation and diversity in FG and NFG output.
The ANM scheme restricts the output of RNFG units when thermal limits on the distribution network are breached and performs preventive and corrective control on the RNFG units through output regulation and tripping. These actions are informed by real time measurement of primary system parameters with the goal to maintain current flow within acceptable and safe limits defined by operating margins. More details of this scheme can be found in the following publications: R. A. F. Currie, G. W. Ault, D. Telford; “Facilitate Generation Connections on Orkney by Automatic Distribution Network Management”; DTI Project Final Report, contract: K/EL/00311/00/00, URN: 05/514, 2005 and R. A. F. Currie, G. W. Ault, J. R. McDonald; “Methodology for the Determination of the Economic Connection Capacity for Renewable Generator Connections to Distribution Networks Optimised by Active Power Flow Management”; IEE Proceedings, Generation, Transmission and Distribution, May 2006, and G. W. Ault, R. A. F. Currie, J. R. McDonald; “Active Power Flow Management Solutions for Maximising DG Connection Capacity”, IEEE PES General Meeting, Montreal, Invited Panel Paper, 2006.
Although ANM schemes are known, much of the work in this area has been theoretical and does not address the design, operation and deployment of an ANM scheme to electricity networks.